The present invention relates to a method and apparatus for separating liquids by fractional distillation.
The present invention will be described with particular reference to the refining of crude oil. However, the apparatus and method of the present invention may be used to separate liquids from mixtures other than crude oil and no limitation is intended thereby.
Fractional distillation is used to separate liquid mixtures according to the boiling range of the components. In oil refineries crude oil is separated into desired liquid fractions such as diesel, fuel oil and gasoline by fractional distillation. Fractional distillation is typically carried out in a distillation column. The liquid mixture is introduced into the column in vaporised form. The vapour rises against a descending stream of condensed liquid. Packing material such as perforated trays are provided in the column which increase the surface area to allow intimate contact between rising vapour and descending liquid.
A temperature gradient exists between the hotter lower portions of the vessel and the cooler upper portions. This temperature gradient may also be facilitated by introducing a cooling liquid to the upper portion of the column. A liquid/vapour equilibrium is present at each level of the column. At points in the column at which the temperature corresponds to the temperature of a desired product liquid is drawn from the tower. Since light products must pass through heavier products before reaching the relevant drawoff point, liquids which are drawn off at intermediate points in the column invariably contain such lighter products. These lighter products must consequently be removed by stripping.
In order to obtain effective separation, distillation columns must be quite high and are typically about 20 to 80 metres. The columns must be constructed so as to resist wind loads and forces generated by earthquakes. This adds considerably to the cost. There is also a safety risk in providing personnel access to the higher parts of the column as is required for regular maintenance. Still further, it is often desirable to refine crude oil on sites adjacent ocean oil platforms. On fixed oil or gas platforms, the height of a conventional column may represent a hazard for on-board cranes and service helicopters. When high columns are mounted to sea going vessels they are subject to large forces and horizontal position shift induced by the vessels roll and pitch. This movement disturbs the mix of the vapour and liquid streams and reduces the efficiency of the separation.
It is therefore desirable to be able to provide a column for fractional distillation which has a reduced height. This has been accomplished in the past by providing a number of separate columns connected in series. In order to provide the necessary counter flow of rising vapour and descending liquid such systems pump hot liquid from the bottom of one column to the top of the previous column. Additionally vapour from the top of one column must be passed to the bottom of the next column. This flow of liquid and vapour between columns requires extensive piping and pumping equipment. Further, the liquid and vapour which are transferred between columns are very hot. This requires the use of high temperature pumping equipment.
It is therefore an object of the present invention to provide a method and apparatus for fractional distillation which may at least partially overcome the above disadvantages or provide the public with a useful choice.
According to a first broad form of the invention there is provided an apparatus for separating a liquid mixture by fractional distillation, the apparatus including at least two chambers which are in vapour and fluid communication and liquid flows from the lower portion of one chamber to the upper portion of an adjacent chamber, wherein said flow of liquid to the adjacent chamber is effected by introducing a pressurised vapour into the liquid.
According to a second broad form of the invention there is provided a method of transferring liquid between chambers in a fractional distillation apparatus, said transfer being from the lower portion of one said chamber to an upper portion of an adjacent said chamber, the method including introducing a pressurised vapour into the liquid so as to effect said transfer.
The apparatus of the present invention may be used to distil any suitable mixture and is particularly suitable for the distillation of crude oil.
The apparatus includes two or more chambers which are in liquid and vapour communication. Therefore, in use there is a transfer of vapour and liquid between chambers. This produces an upflow of vapour and a downflow of liquid in any one chamber.
The mixture to be separated is typically introduced into a first chamber as a vapour. The apparatus includes two or more chambers. Typically, a coolant liquid is introduced into the final chamber. This produces a desired temperature gradient throughout the apparatus. Additional cooling may be provided by introducing a liquid into an upper portion of individual chambers. A similar addition of liquid is employed at selected heights in tower distillation columns. This is known as a side reflux. Typically, the introduced liquids are the same liquids as those which are being withdrawn at that particular height of a single column or from that particular chamber in the apparatus of the present invention.
As in the known single columns, when the temperature corresponds to the temperature of a desired liquid fraction, that liquid fraction is drawn off. In the apparatus of the present invention, a desired liquid fraction is typically drawn off from a lower portion of a respective chamber. Thus, the number of chambers typically corresponds to the number of desired liquid fractions. A typical apparatus for distilling crude oil would include three chambers. The liquid products withdrawn from the respective chambers being fuel oil, diesel and jet fuel.
The liquid is caused to flow between a lower portion of one chamber to an upper portion of an adjacent chamber by the introduction of a pressurised vapour. Typically, the vapour is introduced into the liquid in the lower part of the chamber. The pressurised vapour produces a two phase mixture of gas and liquid. When this occurs in a confined space, the density of the liquid is reduced. This principle is known as the Pohle air lift. In the present invention the effect of the Pohle air lift is to pump liquid from the lower portion of one chamber towards or to an upper portion of a previous chamber. Vapour flows in the reverse direction from the upper portion of one chamber to the lower portion of the next chamber.
In a preferred embodiment of the present invention the pressurised vapour is injected into the system. The vapour may be injected by using a jet injector pump with a venturi nozzle. Such pumps are known. The vapour is introduced at a high velocity. The kinetic energy from the injected vapour assists in forcing the liquid to the upper portion of the previous chamber.
The use of an injection pump may assist the action of the Pohle air lift in pumping the liquid. Such assistance may allow chambers of a greater height to be used. Further, in some cases, a relatively large amount of vapour necessary to be introduced to the liquid to create the Pohle air lift may be required. This may be undesirable, particularly when the vapour is a distillation product. Introducing the vapour by using an injection pump or similar device may enable the amount of vapour to be reduced.
In an alternative embodiment of the present invention, the vapour may be introduced into the upper portion of the flow of liquid to the previous chamber. Introduction of the vapour reduces the pressure and effectively creates a vacuum in the upper portion of the liquid flow. This vacuum draws the liquid upwards. Injection of vapour into the upper portion of the liquid flow may be used on its own or in addition to introducing the vapour into the lower liquid level.
The pressurised vapour which is introduced to the liquid may be any suitable vapour. The vapour may be a revaporised by-product of the distillation such as vaporised naptha. Alternatively, an inert vapour such as steam may be used. In the event that steam or other inert vapour is used then the use of a surplus lift pump capacity, with a portion of the xe2x80x9cliftedxe2x80x9d product allowed to recycle back to the source segment bottom, will give rise to a xe2x80x9cstrippingxe2x80x9d action and remove the small quantities of entrained light product in the withdrawn product from the segment. This may avoid the use of separate external xe2x80x9cstripperxe2x80x9d vessels conventionally employed for this purpose.
The chambers may be arranged in any suitable configuration. The chambers may be discrete units connected by pipes for transfer of vapour and liquid. Preferably, the chambers are housed in a single vessel. In a preferred embodiment of the invention the apparatus includes a vessel of circular cross section in which the chambers are separated by radial partitions. Alternatively, the chambers may be in the form of discrete units housed within the single vessel. This allows the chambers to be manufactured independently and then installed into the single vessel. It will also be appreciated that such an arrangement does not require the individual chambers to be able to withstand internal pressures. The outer vessel, only, is required to be able to operate under pressure. The vessel is typically of circular cross section which optimises the internal pressures it can withstand. The apparatus includes conduits or subchambers for the transfer of fluids and vapour. Preferably, the partitions are constructed of a material to minimise heat transfer between chambers. Preferably the vessel partitions are therefore constructed of stainless steel.
By way of Example only, the present invention will be described by reference to the following Figures in which;